Mobile radios traditionally operate within a defined frequency band and within certain predefined channels in the frequency band given by the particular applications of the radios. For example, a mobile radio used in a cellular radio system will be connected to other mobile radios in the cellular radio system at an assigned channel of, for example (in North America), a 30 KHz band between 824 MHz and 849 MHz for transmit frequencies (or 869 MHz to 894 MHz) for receive frequencies. This results in 832 possible channels within the assigned frequency bands.
As is well known, geographical areas for cellular radio communications are divided into cells. Common practice is to assign a certain number of the 832 possible channels (less any control channels) to each cell, for example 10-50 per cell. The number of channels used per cell depends, of course, on the traffic loads, etc. As mobile radios move from cell to cell, they are considered to be "roaming" and may change channels as they do so.
Other mobile radio applications will use different channel bands at different frequencies. Land mobile radio bands, for example, currently broadcast on channels that are 25 KHz wide. GSM (Ground System Mobile-European Cellular System) and PCS (Personal Communication System) can operate at still different channel frequency bands over different total system bandwidthes. An example of a mobile radio roaming through various different types of mobile radio frequencies plans is shown in FIG. 1. There, the mobile radio is shown as potentially existing in Land Mobile Radio (LMR), Cellular, Ground System Mobile (GSM), and Personal Communication System (PCS) networks.
The different channel frequency bands and the different system bandwidths that characterize different mobile radio systems are referred to as frequency plans. The different frequency plans that exist for different mobile radio systems cause common mobile radios to be practically un-usable across the multiple plans. Thus, traditionally, radios are operational within a fixed and unique frequency plan, such as North American Cellular or Land Mobile, but not both. Since the different frequency plans are separated by constant, fixed-size channel frequency steps of different length, the radios were designed to accommodate one such step length, but not the other (without expensive, impractical modifications).
Thus, for example, the North American Cellular system using 30 KHz steps will employ channels having frequencies of n.multidot.30 KHz, where n is an integer. On the other hand, the Land Mobile Radio bands, employ channels having frequencies of n.multidot.25 KHz, where n is an integer. Radios include circuitry that allow them to step through the total frequency band in the integer increments of the channel widths (i.e. integer multiples of 30 KHz) of the frequency plan in which they are intended to operate.
A radio would increase its flexibility and therefore its marketability if it could comply with non-integer (n&lt;1) frequency steps and thus operate within multiple frequency plans. The two traditional solutions to this problem have proven to be impractical. The first solution was to employ a multiple loop synthesizer architecture within the mobile radio. In these radios, one synthesizer was employed per frequency plan thus increasing expense and complexity even though the multiple synthesizer shared a common reference oscillator. The other solution employed dividing the reference frequency into high common-denominators for the step sizes desired. But, this approach resulted in performance degradation due to the introduction of large spurious signals.